This invention relates to gas turbines, and more particularly to a hybrid sensor for measuring turbine blade tip clearance using radio frequency and ultrasonic transducers.
Increased efficiency in gas turbines is desirable. Measuring the clearance between moving rotor blades and stationary shrouds in compressor and turbine sections of gas turbines is desired as the efficiency of a gas turbine engine is dependent upon, inter-alia, the clearance between tips of its blades and turbine casing. Thus, the smaller the clearances the lower is the gas leakage across the airfoil tips. However, under certain engine conditions, airfoils and their associated discs may experience thermal growth, thus increasing the risk of contact with the casing.
Accurate measurement of clearance between the tips of rotating blades and their associated casing is essential for gas turbine engine development. Prior approaches and systems for providing such measurement include a probe mounted on a casing surrounding the blades. The blade tip and the probe act as two plates of a capacitor, with the probe relying on a capacitance change within the clearance between the blade and casing/probe to measure blade tip clearance.
More specifically, the capacitive probe forms a part of an electric circuit, to which a high frequency electric signal is provided by means of an oscillator. The high frequency electric signal is amplitude modulated by changes in electrical capacitance as the blade tips pass near the probe. The gap between the probe and the blade tips is calculated, based upon the changes in signal amplitude, to determine blade tip clearance. Unfortunately, the capacitive measurements are adversely influenced by the presence of nearby electric or magnetic fields as well as the changes in the water content of gases passing between the blade tips and the probe. A further drawback of this approach is that the probe must be located close to the turbine hot section, thus limiting the life of the probe.
In another approach, the voltage of an electric spark is used to measure the distance between an electrode (probe) and the blade tips. Here, the measurements are adversely affected by changes in the pressure and/or water content of the gases passing between the blade tips and the probe. The electric spark may also cause damage to the passing blade tips.
In yet another approach, observed changes in magnetic field strength, as the blade tips pass near a magnetic probe, are proportional to the gap between the probe and the blade tips. This method is typically not used in gas turbines due to poor frequency response when compared to capacitive techniques. This approach is also subject to limitations listed for other approaches as identified above.
Optical techniques have also been developed for measuring turbine blade tip clearance. Although optical techniques have the advantage that they can be used in the presence of changing electrical or magnetic fields while achieving a faster response time, these optical techniques are susceptible to dirt or oil deposits on the optical system, thus deteriorating the image quality.
Thus, there is a need to overcome the problems identified with prior approaches in order to measure the blade tip clearance (i.e., the distance between a blade tip and the stator shroud surface).
Accordingly, the present invention relates to a method and apparatus for determining the distance between rotating blade tips and a stator shroud surface by utilizing two different sources of energy and associated signal-processing circuitry.
Radio frequency (RF) energy and RADAR signal processing technology are used for measuring the distance from a fixed transducer to a moving metallic blade. This technique thus offers an advantage of locating a sensor transducer and associated electronics far from the turbine hot section, thereby increasing sensor life. A ceramic or other non-conducting material would be used as a waveguide to direct the RF energy to and from the transducer. The RF energy is reflected from the passing blades, and the distance between the blade tip and the RF transducer can thus be calculated. However, using the above approach, the distance between the blade tip and the stator shroud surface is not measured.
Specifically, the clearance between moving rotor blades and stationary shrouds in the compressor and turbine sections of a gas turbine is achieved by using a hybrid sensor apparatus. RF energy is used to measure the distance between the blade tip and the RF transducer as described above, and ultrasonic energy is used to measure the distance between the stator shroud and the ultrasonic (UT) transducer. The RF transducer and UT transducer are preferably located at equal radial distances from the turbine centerline. The distance between the stator shroud surface and the UT transducer is subtracted from the distance between the blade tip and the RF transducer to determine the blade tip clearance. The subtraction is preferably performed using electronic circuitry.
In its broader aspects, the present invention provides a method for determining blade tip clearance in a gas turbine, the method comprising (a) measuring a distance between an ultrasonic sensor and a stator shroud surface; (b) measuring a distance between a radio frequency sensor and rotating blade tips of the gas turbine; and (c) using the distance measured in step (a) and the distance measured in step (b) to determine the blade tip clearance in the gas turbine. Step (a) further comprises using an ultrasonic transducer to direct ultrasonic energy towards the stator shroud surface and to receive reflected ultrasonic energy; and providing a measurement in real-time to a processor. Step (b) further comprises using a radio frequency transducer to direct radio frequency energy towards the rotating blade tips of the gas turbine and to receive reflected radio frequency energy; and providing a measurement in real-time to a processor.
An ultrasonic waveguide is preferably used to direct ultrasonic energy between the ultrasonic transducer and the stator shroud surface. A radio frequency waveguide is preferably used to direct radio frequency energy between the radio frequency transducer and the rotating blade tips of the gas turbine. The radio frequency waveguide is preferably made from ceramic materials. Both the ultrasonic sensor and the radio frequency sensor are preferably disposed at equal radial distances from a reference point on the gas turbine, the reference point being centerline of a rotor.
In another aspect, the present invention provides an apparatus for determining blade tip clearance of a gas turbine, the apparatus comprising: an ultrasonic sensor for measuring a distance between a stator shroud surface and a position of the ultrasonic sensor and providing a first signal indicative thereof; a radio frequency sensor for measuring a distance between rotating blade tips of the gas turbine and a position of the radio frequency sensor and providing a second signal indicative thereof; and a processor for receiving and processing said first and second signals to determine blade tip clearance of the gas turbine. The apparatus further includes an ultrasonic waveguide for directing ultrasonic energy between the ultrasonic sensor and the stator shroud surface, and wherein a first end of the ultrasonic waveguide is fixed to the ultrasonic sensor, and an opposite second end is made an integral part of the shroud surface. The apparatus further includes a radio frequency waveguide for directing radio frequency between the radio frequency sensor and the rotating blade tips of the gas turbine.
In another aspect, a method for determining a distance between a tip of a rotating blade and a stator shroud surface of a gas turbine, the method comprising: (a) measuring a distance between an ultrasonic transducer and the stator shroud surface; (b) measuring a distance between a radio frequency transducer and the tip of the rotating blade of the gas turbine; and (c) using the distance measured in step (a) and the distance measured in step (b) to determine the distance between the tip of the blade and the stator shroud surface.
In a further aspect, the present invention provides a gas turbine having a stator having a stator shroud surface; a plurality of rotating blades with respective blade tips; a first means for measuring the distance between the stator shroud surface and the first means; a second means for measuring the distance between the rotating blade tips and the second means; and means for receiving and using measurements made by the first means and the second means to determine blade tip clearance of the gas turbine. The apparatus further comprises a means for directing ultrasonic energy between the first means and the stator shroud surface, a means for directing radio frequency between the second means and the rotating blade tips of the gas turbine. Preferably, both the first and second means are disposed at equal radial distances from a reference point on the gas turbine, the reference point being centerline of gas turbine rotor.
In a further aspect, a hybrid sensor for determining the clearance between rotor blade tips and a stator shroud of a gas turbine, the hybrid sensor comprising: an ultrasonic sensor for measuring a distance between a stator shroud surface and the ultrasonic sensor; a radio frequency sensor for measuring a distance between rotating blade tips and the radio frequency sensor; and a processor for receiving and using the ultrasonic sensor measurements and the radio frequency sensor measurements to determine blade tip clearance of the gas turbine.
In yet another aspect, a method of operating a gas turbine, the method comprising the steps of: a) using an ultrasonic transducer to generate a first signal indicative of a distance between a stator shroud surface and a position of the ultrasonic transducer; b) using a radio frequency (RF) transducer to generate a second signal indicative of a distance between rotating blade tips and a position of the RF transducer, the position of the ultrasonic transducer and the position of the RF transducer being disposed essentially at equal radial distances from a from a reference point on the gas turbine; and c) using the first signal and the second signal to determine a clearance between the blade tips and the stator shroud surface.
In a further aspect, a method of operating a gas turbine, the method comprising the steps of: a) using a first sensor to generate a first signal indicative of a distance between a stator shroud surface and a position of the first sensor; b) using a second sensor to generate a second signal indicative of a distance between rotating blade tips and a position of the second sensor, the position of the first sensor and the position of the second sensor being essentially radially equidistant from a reference point on the gas turbine; and c) using the first signal and the second signal to determine a clearance between the blade tips and the stator shroud surface.
In another embodiment, the method and apparatus of the present invention is used to measure a blade tip clearance in a steam turbine according to various aspects of the present invention.
In yet another embodiment, the method and apparatus of the present invention is used to measure a blade tip clearance in a compressor according to various aspects of the present invention.
In a further aspect, the present invention provides a hybrid sensor for determining the clearance between moving rotor blades and stationary shrouds in compressor and turbine sections of a gas turbine in accordance with various aspects of the present invention.